Silver halide emulsion

Abstract: A silver halide emulsion comprises silver halide grains, which are dispersed in a dispersing medium. At least 60 % of the silver halide grains based on projected area of the grains are specific tabular grains having an aspect ratio (diameter per thickness) of not less than 2. The specific tabular silver halide grains have a chloride content in the range of 20 to 99.99 mol %. The specific silver halide grains further have {100} major faces. The specific grains furthermore have a thickness of not more than 0.35 µm. According to the present invention, twinned crystal grains contained in the emulsion comprise not more than 10 % of the specific tabular silver halide grains based on projected area of the grains. Further, the specific tabular silver halide grains are formed from grain nuclei. The nuclei have at least one gap interface of halide composition.

Description: FIELD OF THE INVENTION The present invention relates to a silver halide emulsion useful in photography. In more detail, the invention relates to a silver halide emulsion containing tabular silver halide grains that have {100} major faces. BACKGROUND OF THE INVENTION Japanese Patent Provisional Publication No. 51(1976)-88017, Japanese Patent No. 64(1989)-8323 and European Patent Publication No. 0534395A1 disclose silver halide emulsions containing tabular silver halide grains that have {100} major faces. European Patent Publication No. 0534395A1 further discloses high chloride tabular grains, which are shown in photographs of Figures 1 to 5. Referring to the Figures, twinned crystal grains are remarkably observed in the photographs. Further, the grains shown in the photographs have a relatively broad grain size distribution. The Figure 1, which represents statistically reliable number of the grains is now described in more detail. Referring to the grains shown in the Figure 1, the ratio of twinned crystal grains to tabular grains having an aspect ratio of not less than 2 is about 11 % based on projected areas of the grains. Further, the grain size distribution of tabular grains having a thickness of not more than 35 µm is relatively broad. The grain size distribution of relatively large tabular grains is particularly broad, which affects the photographic property of the grains. In more detail, the relatively large grains have a distribution coefficient of more than 0.12, in which the relatively large grain comprises 70 % of the tabular silver halide grains based on projected areas of the grains. Further, the amount of tabular silver halide grains having a square projected shape is relatively small. In more detail, about 40 % of the tabular silver halide grains have a square projected shape that a ratio of a long side to a short side is not more than 1.4. According to study of the present inventor, the above-mentioned characteristics about the grains shown in Figure 1 of European Patent Publication No. 0534395A1 degrade sensitivity and image quality of a silver halide photographic material. Further, Japanese Patent Provisional Publication No. 63(1988)-271335 discloses a silver halide emulsion containing the above-described tabular grains and needle-like grains. SUMMARY OF THE INVENTION An object of the present invention is to provide a silver halide emulsion containing silver chloride, which is improved in sensitivity and image quality. The present invention provides a silver halide emulsion comprising silver halide grains, which are dispersed in a dispersing medium, wherein at least 60 % of the silver halide grains based on projected areas of the grains are tabular grains having an aspect ratio of not less than 2, said tabular silver halide grains having a chloride content in the range of 20 to 99.99 mol %, {100} major faces, and a thickness of not more than 0.35 µm, and wherein twinned crystal grains contained in the emulsion comprise not more than 10 % of the tabular silver halide grains based on projected areas of the grains, and the tabular silver halide grains are formed from grain nuclei, which have at least one gap interface of halide composition. The present inventor has prepared a new silver halide emulsion having the above-defined specific characteristics. The new silver halide grains are superior to conventional tabular silver halide grains having {100} major faces with respect to the sensitivity and the image quality (particularly graininess). BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a micrograph (about × 6,500) showing silver halide grains of the present invention obtained in Example 1. Figure 2 is a micrograph (about × 5,000) showing silver halide grains of the present invention obtained in Example 2. Figures 3a (about × 18,000) and 3b (about × 13,000) are micrographs showing silver halide grains of the present invention obtained in Example 3. DETAILED DESCRIPTION OF THE INVENTION In the present specification, silver halide grains contained in emulsions are evaluated about the following characteristics (1) to (9): (1) A ratio of tabular grains having an aspect ratio (diameter per thickness) of not less than 2 to whole silver halide grains (% based on projected areas of the grains); (2) A chloride content of tabular grains having an aspect ratio of not less than 2 (mol %); (3) Crystallographic planes of major faces of tabular grains having an aspect ratio of not less than 2; (4) A thickness of tabular grains having an aspect ratio of not less than 2 (µm). (5) A ratio of twinned crystal grains to tabular grains having an aspect ratio of not less than 2 (% based on projected areas of the grains); (6) Halide composition of grain nuclei from which tabular grains having an aspect ratio of not less than 2 are formed; (7) A distribution coefficient of relatively large tabular silver halide grains, said relatively large grains comprising 70 % of tabular silver halide grains having an aspect ratio of not less than 2 based on projected areas of the grains; (8) A ratio of specific square tabular grains to tabular silver halide grains having an aspect ratio of not less than 2, said square tabular grains having such a square projected shape that a ratio of a long side to a short side is not more than 1.4 (% based on projected areas of the grains); and (9) A ratio of specific rectangular tabular grains to tabular silver halide grains having an aspect ratio of not less than 2, said rectangular tabular silver halide grains having such a rectangular projected shape that one angle is rounded compared with the other three angles (% based on projected areas of the grains). The photographic emulsion of the present invention is characterized in that: (1) is at least 60 %; (2) is in the range of 20 to 99.99 mol %; (3) are {100} major faces; (4) is not more than 0.35 µm; (5) is not more than 10 %; and (6) are nuclei having at least one gap interface of halide composition. The conditions defined in the present invention are described below in more detail. The condition (1) is calculated by the following formula: (1) = 100 × (total projected area of tabular grains having an aspect ratio of not less than 2 per total projected area of whole silver halide grains) According to the present invention, the condition (1) is at least 60 %, preferably is at least 70 %, and more preferably is at least 80 %. The aspect ratio means diameter per thickness. The diameter (d) is calculated according to the following formula; d = 2 × S π (S: total projected area of grain) The thickness means a distance between two major planes of a tabular grain. The condition (2), namely the chloride content of the tabular grains is in the range of 20 to 99.99 mol %, preferably in the range of 50 to 99.9 mol %, more preferably in the range of 75 to 99.9 mol %, and most preferably in the range of 90 to 99.9 mol %. With respect to the condition (3), the tabular grains have (100) major faces. The major face means the largest crystal plane of a silver halide grain. The condition (4), namely the thickness of the tabular grains is not more than 0.35 µm, preferably in the range of 0.05 to 0.3 µm, and more preferably in the range of 0.05 to 0.25 µm. Additionally, the tabular silver halide grains preferably have an average aspect ratio in the range of 2 to 25, and more preferably in the range of 4 to 20. The condition (5) is calculated by the following formula: (5) = 100 × (total projected area of twinned crystal grains per total projected area of tabular grains having an aspect ratio of not less than 2). According to the present invention, the condition (5) is in the range of 0 to 10 %, preferably is in the range of 0 to 8 %, more preferably is in the range of 0 to 5 %, and most preferably is in the range of 0 to 2 %. The condition (6) is described below with respect to preparation of silver halide grains. The photographic emulsion of the present invention preferably has at least one of the following characteristics (7) to (9): (7) is in the range of 0 to 0.11; (8) is at least 47 %; or (9) is at least 10 %. The condition (7) is evaluated in the following manner: (7a) Tabular silver halide grains having an aspect ratio of not less than 2 are selected from silver halide grains; (7b) Projected areas of the tabular silver halide grains are measured; (7c) The tabular grains are drawn up in order of the size of the projected area; (7d) Grains having a large projected area are selected in the order from the tabular grains until total projected area of the selected large grains comprises 70 % of total projected area of the whole tabular grains; (7e) Diameters (d) of the selected large tabular grains are calculated according to the following formula; d = 2 × S π (S: total projected area of grain) (7f) Average diameters ( d ) of the selected large tabular grains are calculated according to the following formula; and (di: individual diameters calculated as above, n: number of calculated grains) (7g) Standard deviation (σ) of the selected large tabular silver halide grains calculated according to the following formula; and (7h) Distribution coefficient (σ/ d ) is obtained from the average diameter ( d ) and the standard deviation (σ). The condition (7) is preferably in the range of 0 to 0.11, more preferably in the range of 0 to 0.09, and most preferably in the range of 0 to 0.06. The condition (8) is calculated by the following formula: (8) = 100 × (total projected area of the specific square tabular grains per total projected area of tabular silver halide grains having an aspect ratio of not less than 2). The condition (7) is preferably at least 47 %, more preferably at least 50 %, further preferably at least 55 %, furthermore preferably at least 75 %, and most preferably at least 85 %. With respect to the specific square tabular grains, the ratio of a long side to a short side is in the range of 1 to 1.4, preferably is in the range of 1 to 1.3, and most preferably in the range of 1 to 1.2. With respect to the square shape, a corner of the square can be rounded to some extent. The ratio of the volume defected by the rounded corner to the rectangular prism volume is preferably not more than 0.2, and more preferably is not more than 0.1. The rectangular prism volume means a volume of a rectangular prism, which is theoretically formed by extending edges of the grain. In a high chloride emulsion of the present invention, one corner tends to be primarily rounded, as is defined in the condition (9). The condition (9) is calculated by the following formula: (9) = 100 × (total projected area of the specific rectangular tabular grains per total projected area of tabular silver halide grains having an aspect ratio of not less than 2). The condition (9) is preferably at least 10 %, more preferably at least 15 %, and most preferably at least 35 %. The specific rectangular tabular grains have such a rectangular projected shape that one angle is rounded compared with the other three angles. In other words, one corner of the rectangular is primarily rounded. The volume defected by the primarily rounded corner is preferably twice or more, more preferably 4 times or more, and most preferably 8 times or more as much as the volume defected by another corner. Additionally, the tabular silver halide grains preferably have an average diameter in the range of 0.2 to 10 µm, and more preferably in the range of 0.3 to 5 µm. The iodide content of the tabular silver halide grains preferably is not more than 13 mol %, more preferably is not more than 6 mol %, and most preferably in the range of 0 to 3 mol %. The tabular silver halide grains in the emulsion of the present invention are formed from grain nuclei, which have at least one gap interface of halide composition according to the condition (6). The tabular grains are grown primarily along the direction of edges. The primary growth is promoted by a defect in grain nuclei. A defect in grain nuclei for forming a tabular grain is referred to as a spiral dislocation defect. The defect is formed by at least one (preferably 1 to 3 and more preferably 1 or 2) gap interface of halide composition in the nuclei. The defect is preferably formed by placing a layer of a silver halide having a low solubility (AgX 2 ) on a layer of another silver halide (AgX 1 ) having a high solubility. Accordingly, the gap interface of halide composition is effectively formed by a halide conversion reaction. The solubility of silver halide decreases in the order of AgCl, AgBr and AgI. Accordingly, a silver halide having a higher chloride and a lower iodide has a higher solubility. For example, a halide composition in the grain nuclei has a structure of AgX 1 |AgX 2 or AgX 1 |AgX 4 |AgX 3 . The structure can be formed by mixing simultaneously a silver (Ag + ) solution with a halide (X-) solution, and changing discontinuously a halide composition of the halide solution at the gap interface. Further, a halide solution and a silver solution can be added to a solution of a dispersing medium to form AgX 1 , and then another halide solution and a silver halide solution can be added to the medium to form a structure of AgX 1 |AgX 2 . The above-mentioned methods can be used in combination. In the structure of AgX 1 |AgX 2 or AgX 1 |AgX 4 |AgX 3 , a difference in chloride content or bromide content between AgX 1 and AgX 2 , AgX 1 and AgX 4 or AgX 4 and AgX 3 is preferably in the range of 25 to 100 mol %, more preferably in the range of 50 to 100 mol %, and most preferably in the range of 75 to 100 mol %. Alternatively, a difference in iodide content is preferably in the range of 5 to 100 mol %, more preferably in the range of 10 to 100 mol %, and most preferably in the range of 30 to 100 mol %. There can be differences in the chloride or bromide content and the iodide content. In such a case, the difference in the chloride or bromide content is defined above, and the difference in iodide content is preferably in the range of 0 to 5 mol %. The grain nuclei preferably have a diameter of not more than 0.15 µm, and more preferably in the range of 0.01 to 0.1 µm. In the structure of AgX 1 |AgX 2 or AgX 1 |AgX 4 |AgX 3 , the molar ratio of AgX 1 :AgX 2 or AgX 1 :AgX 4 :AgX 3 can be experimentally determined to obtain the optimum embodiment of the present invention. In the structure of AgX 1 |AgX 2 , the layer of AgX 2 preferably has such a thickness that one lattice of AgX 2 on the average covers the surface of AgX 1 . The thickness of the AgX 2 layer is more preferably in the range of three lattices on the average to 10 4 mol of the AgX 1 layer. In the structure of AgX 1 |AgX 4 |AgX 3 , the amount of the AgX 4 layer is preferably in the range of 0.01 to 10 mol, and more preferably in the range of 0.1 to 3 mol based on one mole of the AgX 1 layer. The defect in grain nuclei is more frequently formed by increasing the difference of the gap in a halide content. The structure of AgX 1 |AgX 4 |AgX 3 can also be formed by forming AgX 1 nuclei, adding an X 4 - salt solution to the nuclei to form a structure of AgX 1 |AgX 4 according to a halide conversion reaction, and then a Ag + salt solution and an X 3 salt solution to form the nuclei. The X 4 - salt preferably satisfies the condition that the solubility of AgX 4 is lower than that of AgX 1 . The amount of X 4 - contained in the solution is preferably in the range of 0.01 to 10 mol, and more preferably in the range of 0.03 to 3 mol based on one mole of AgX 1 . As the amount of AgX 4 - based on the amount of AgX 1 can be reduced with increasing the gap difference in the halide composition. In the most preferred embodiment of the present invention, the differences in chloride content and bromide content are in the range of 25 to 100 mol %, more preferably in the range of 50 to 100 mol %, and most preferably in the range of 75 to 100 mol %. The structure of AgX 1 |AgX 4 |AgX 3 is particularly preferred. When the nuclei are formed, the atmosphere of the dispersing medium solution is adjusted to form {100} faces. Where the nuclei are formed in the presence of an excess amount of chloride, usual conditions (pCl: 0.8 to 3.0, pH 2 to 8) are under the atmosphere for forming {100} faces. The frequency of forming the defect is increased by increasing pH (in the range of 1 to 7) and pCl (= -log[Cl-mol per liter]). When many defects are introduced into a silver halide grain, thick grains are increased in the finally obtained silver halide emulsion. Accordingly, the number of the defects are appropriately adjusted. The conditions for forming the defects are adjusted to obtain the silver halide emulsion of the present invention. The gap interface is uniformly formed in each of the nuclei to obtain the embodiment of the present invention. A dispersing medium for forming nuclei is preferably used in the form of 0.1 to 10 wt.% solution, and more preferably 0.3 to 5 wt.% solution. The solution of the dispersing medium is preferably adjusted to pH 1 to 10, and more preferably to pH 2 to 8. The temperature of the solution is preferably adjusted to 10 to 80 °C, and more preferably to 30 to 60 °C. When the temperature is 30 or lower °C, the frequency of forming the defect is decreased. A certain temperature is necessary for forming the defect. The excess bromide content is preferably not more than 10 -2 mol per liter, and more preferably not more than 10 -2.5 mol per liter. The excess chloride content is preferably in the range of 0.8 to 3.0 pCl, and more preferably in the range of 1.2 to 2.8 pCl. The dispersing medium can be added to a silver salt solution and/or a halide salt solution to form uniform grain nuclei. The concentration of the dispersing medium in the silver salt or halide salt solution is preferably not less than 0.1 wt.%, more preferably in the range of 0.1 to 2 wt.%, and most preferably in the range of 0.2 to 1 wt.%. The dispersing medium preferably is gelatin, which more preferably has a molecular weight in the range of 3,000 to 60,000, and most preferably in the range of 8,000 to 40,000. The silver salt solution and the halide solution are preferably added through a porous addition system. The system preferably has 3 to 10 15 pores, and more preferably has 30 to 10 15 pores. The porous system is described in Japanese Patent Provisional Publications No. 3(1991)-21339 and No. 4(1992)-193336, and Japanese Patent Application No. 4(1992)-40283. The frequency of forming defect is increased by decreasing the methionine content in gelatin. Gelatin is preferably selected in consideration of the methionine content in the range of 1 to 60 µmol per 1 g of gelatin. A ratio of twinned crystal grains can be decreased by lowering the excess halide salt concentration or the excess silver salt concentration when the grain nuclei are formed. The ratio is increased by lowering the concentration of the dispersing medium or degrading stirring conditions. The conditions can be experimentally determined to obtain silver halide grains of the present invention. A spiral dislocation defect is formed by forming the gap interface of a halide composition and a halide conversion on the interface when the grain nuclei are formed. After the defect is formed, the silver halide grains are ripened when the temperature is elevated at least 10 °C, and more preferably 20 to 70 °C. The ripening atmosphere is preferably adjusted to form {100} faces. The ripening conditions are selected from the above-mentioned conditions for the grain nuclei formation. In the ripening, tabular grains are primarily grown and non-tabular grains are decreased. Thus the ratio of the tabular grains is increased. The ripening speed can be increased by increasing pH in the range of 1 to 6 and increasing Cl - content in the range of 1 to 3 pCl. After the ratio of the tabular grains is increased, solutes are added to the grains to further grow the tabular grains. The solutes can be added according to an ion addition method (adding Ag + solution and X - solution), a fine grain addition method (adding silver halide fine grains, which are previously formed) or a combination thereof. The tabular grains are grown primarily along the direction of the edges under the conditions that the concentration of supersaturation is lowered so long as the tabular grains are not subjected to Ostwald ripening. Accordingly, the concentration should be precisely adjusted to a low supersaturation concentration The fine grain addition method is preferably used to adjust the concentration precisely. The method is also advantageous to grow the tabular grains uniformly. In the fine grain addition method, fine silver grains have an average particle size of not more than 0.15 µm, preferably not more than 0.1 µm, and more preferably in the range of 0.06 to 0.006 µm. The fine grains are subjected to Ostwald ripening for grow the tabular grains. The fine grains are continuously or discontinuously added to the emulsion. For example, the fine grains can be continuously formed by mixing AgNO 3 solution and an X - salt solution in a vessel placed near a reaction vessel, and immediately and continuously added to the reaction vessel. Further, the fine grains can be formed in another vessel according to a batch method, and continuously or discontinuously added to a reaction vessel. The fine grains are used in the form of an emulsion or a dry powder. The dry powder of the fine grains can be dispersed in water prior to addition. The conditions in the fine grain addition method are preferably so adjusted that the fine grains are dissolved within 20 minutes, more preferably within 10 seconds to 10 minutes. When the time for dissolving the fine grains is increased, the grains are ripened and the grain size is increased. Accordingly, the time for dissolving the fine grains is preferably shortened. For example, the grains are gradually added to the reaction vessel. It is preferred that the fine grains substantially do not contain multiple twinned crystal grains. The multiple twinned crystal means a grain having two or more twinned plane. It is also preferred that the fine grains substantially do not contain single twinned crystal grains. It is further preferred that the fine grains substantially do not have spiral dislocation. The term "substantially do not contain (or have)" means that the ratio of the specific grains is not more than 5 % (preferably not more than 1 %, and more preferably not more than 0.1 %). The halide composition of the fine grains preferably is AgCl, AgBr, AgBrI (wherein I - content preferably is not more than 10 mol %, and more preferably is not more than 5 mol %) or a mixture thereof (mixed crystals). The halide compositions of the fine grains are described in Japanese Patent Application No. 4(1992)-214109. Various dispersing mediums for a conventional silver halide emulsion are available for forming grain nuclei, grain ripening and grain growth. Gelatin is preferably used as the dispersing medium. The methionine content of gelatin is preferably in the range of 0 to 50 µmol, and more preferably in the range of 0 to 30 µm. When gelatin having a low methionine content is used in grain ripening or grain growth, thin tabular grains having a uniform grain size distribution are formed. Such tabular grains are preferred. Further, a synthetic polymer is available as the dispersing medium. The synthetic polymers are disclosed in Japanese Patent Publication No. 52(1977)-16365, Journal of Japan Photographic Society, volume 29 (1), 17, 22 (1966), volume 30 (1), 10, 19 (1967), volume 30 (2), 17 (1967) and volume 33 (3), 24 (1967). An agent for adjusting crystal habit is also available. The adjusting agent is disclosed in European Patent Publication No. 0534395A1. The concentration of the dispersing medium is preferably in the range of 10 -1 to 10 -6 mol per liter. The concentration of the adjusting agent is preferably in the range of 10 -1 to 10 6 mol per liter, and more preferably in the range of 10 -2 to 10 5 mol per liter. The medium and agent can be added at any stages from the nuclei formation to the grain growth. A dispersing medium can be used in addition to the medium previously present in an emulsion. A dispersing medium can be also be added to the emulsion after the medium previously present is removed from the emulsion. In the grain ripening and grain growth, the concentration of the dispersing medium, pH, the concentration of X-salt solution, the addition methods of Ag + solution and X-solution (porous addition system and dispersing medium) can be adjusted by referring to the conditions for the nuclei formation, which are described above. The temperature for the grain ripening and grain growth is preferably not lower than 25 °C, and more preferably in the range of 30 to 80 °C. The atmosphere of the grain growth is also preferably adjusted to form {100} faces. The atmosphere for forming {100} faces in the nuclei formation, the grain ripening or the grain growth are so adjusted that 60 to 100 % (preferably 80 to 100 %, and more preferably 90 to 100%) of the grain surface become {100} faces. The ratio of the {100} faces can be measured, for example by a method described in T. Tani, Journal of Imaging Science, volume 29, 165 (1985). The silver halide emulsion of the present invention can be prepared by another method. For examples, the grains can be formed by (a) nuclei formation, (b) selectively growing tabular grain under a low supersaturated concentration to enlarge the size difference between the tabular grains and non-tabular grains and (c) dissolving the non-tabular grains by Ostwald ripening. The grains can also be formed by (a) nuclei formation and (b) growing tabular grain under conditions of a supersaturated concentration and Ostwald ripening at a temperature elevated 10 °C or more. As is described above, the tabular grains are grown while non-tabular fine grains are dissolved. The tabular grains mainly contain silver chloride. Various halide compositions can be introduced into the grain structures. For example, the grains have a multiple structure, which is described in Japanese Patent Application No. 5(1993)-96250. The grain structures include embodiments having a higher Br - and/or I - content on the surface of the grains, along the edges of the grains and on the major planes of the grains. The high Br- content is preferably not less than 1 mol % higher than the Br - content in the other portion, more preferably of 3 to 100 mol % higher, and most preferably 5 to 70 mol % higher. The high I - content is preferably not less than 1 mol % higher than the I - content in the other portion, more preferably 2 to 30 mol % higher, and most preferably 3 to 10 mol % higher. The tabular grains have {100} major faces. On the major faces, Ag + and X - are alternatively arranged. When a spectral sensitizing dye is adsorbed on the faces, the dye can directly be reacted with Ag + . In the silver halide grains, Ag + forms a conductive band, and X - forms an electron charged band. Accordingly, the lowest vacant level of the dye can directly be reacted with the conductive band of Ag + . Therefore, the efficiency of the electron injection is high when the dye is excited with light. Thus, the efficiency of spectral sensitization is improved. On the other hand, a sensitizing dye adsorbed on tabular grains having {111} major faces is reacted with the conductive band of Ag + through an X - layer. The surface X - layer has an excess minus charge because the chemical bond is broken on the surface. The electron is injected over the barrier of the minus charge. Therefore, the efficiency of the electron injection is low. Thus, the efficiency of spectral sensitization is low when tabular grains having {111} major faces are used. For the reasons mentioned above, the silver halide grains having {100} major faces are superior to the conventional grains having {111} major faces. Further, the conventional tabular grains having {111} major faces has a tendency of inherent desensitization. When the conventional grains are excited with light within the inherent region, the formed positive hole tends to be captured by the highest occupied orbit of the dye through the X - layer. The captured positive hole is combined again with the excited electron to decrease the inherent desensitization. The positive hole is easily trapped with the X - layer of the {111} major faces. Such an inherent desensitization is scarcely caused in the silver halide grains having {100} major faces. Epitaxial grains can be formed from the obtained silver halide grains. Further, grains having dislocation lines inside the grains can be formed by using the obtained grains as grain nuclei. Further, the obtained grains can be used as substrate of other grains, which are formed by forming silver halide layer on the substrate. The halide composition of the layer is different from that of the substrate. The other various silver halide grains can be formed from the grains used in the present invention. A speck of chemical sensitization is usually introduced into the silver halide grains. The place and number (per cm 2 ) of the chemical sensitization specks are preferably adjusted. The adjustment is described in Japanese Patent Provisional Publications No. 64(1989)-74540, No. 1(1989)-201651, No. 2(1990)-838, No. 2(1990)-146033 and No. 3(1991)-121445 and Japanese Patent Application No. 3(1991)-73266, No. 3(1991)-140712 and No. 3(1991)-115872. A shallow internal latent image emulsion can be prepared by using the tabular grains as the core. A core/shell emulsion can also be prepared. These emulsions are described in Japanese Patent Provisional Publications No. 59(1984)-133542 and No. 63(1988)-151618, and U.S. Patents No. 3,206,313, No. 3,317,322, No. 3,761,276, No. 4,269,927 and No. 3,267,778. The silver halide emulsion of the present invention can be mixed with one or more other emulsions. The mixing ratio of the emulsion of the invention to other emulsions can be appropriately arranged within the range of 1.0 to 0.01. There is no specific limitation with respect to additives, which are used from grain formation to emulsion coating. Examples of the additives include silver halide solvents, doping agents for silver halide grains (e.g., metal compounds of the group VIII or other metals, chalcogen compounds and SCN compounds), dispersing agents, antifogging agents, spectral sensitizing dyes (e.g., blue, green, red, infrared, panchromatic or orthochromatic sensitizers), supersensitizers, chemical sensitizers (e.g., sulfur sensitizers, selenium sensitizers, tellurium sensitizers, gold or other noble metals such as metals of the group VIII sensitizers, phosphor compounds, rhodan compounds or reduction sensitizers), fogging agents, emulsion precipitating agents, surface active agents, hardening agents, dyes, color image forming agents, additives for color photography, soluble silver salt, latent image stabilizer, developing agents (e.g., hydroquinone compounds), agents for preventing pressure desensitization and matting agents. Two or more additives (e.g., chemical sensitizers) can be used in combinations. The silver halide grains and emulsion of the present invention are available in various conventional photographic materials. Examples of the photographic materials include black and white photographic materials (e.g., X-ray sensitive materials, light-sensitive materials for printing, photographic papers, negative films, microfilms, direct positive light-sensitive materials, ultrafine-grain dry light-sensitive materials (for LSI photomask, shadow mask or liquid crystal mask) and color photographic materials (e.g., negative films, color photographic papers, reversal films, direct positive color light-sensitive materials, silver dye bleaching photographic materials). The silver halide emulsion is also available in diffusion transfer light-sensitive materials (e.g., color dye diffusion transfer element, silver salt diffusion transfer element), thermal developable (black and white or color) light-sensitive materials, high density digital recording light-sensitive materials and holography light-sensitive materials. In the photographic material, the coating amount of silver is preferably not less than 0.01 g/m 2 . There is no specific limitation with respect to the structure of the photographic material (e.g., the layered structure, the molar ratio of silver to coloring agents, the silver amounts in the layers), the conditions for light exposure and development, apparatus for preparing the photographic material and dispersing methods of additives. Various conventional structures, additives, methods, apparatus and conditions are available. The conventional structures, additives, methods, apparatus and conditions are described in Research Disclosure, volume 176, item 17643 (December 1978), and volume 307, item 307105 (November 1989); Duffin, Photographic Emulsion Chemistry, Focal Press, New York (1966); E.J. Birr, Stabilization of Photographic Silver Halide Emulsions, Focal Press, London (1974); T.H. James, The Theory of Photographic Process, fourth edition, Macmillan, New York (1977); P. Glafkides, Chimie et Physique Photographiques, fifth edition, Edition de l'usine Nouvelle, Paris (1987), and second edition, Paul Montel, Paris (1957); V.L. Zelikman et al., Making and Coating Photographic Emulsion, Focal Press (1964); K.R. Hollister, Journal of Imaging Science, volume 31, pages 148 to 156 (1987); J.E. Maskasky, Journal of Imaging Science, volume 30, pages 247 to 254 (1986), volume 32, pages 160 to 177 (1988), volume 33, pages 10 to 13 (1989); Frieser et al. Die Grundlagen Der Photographischen Prozesse Mit Silberhalogeniden, Akademische Verlaggesellschaft, Frankfurt (1968); Journal of Japan Chemical Society, pages 18 to 27, December (1984); and Journal of Japan Photographic Society, volume 49, pages 7 to 12 (1986), volume 52, pages 144 to 166 (1989), volume 52, pages 41 to 48 (1989). The photographic materials are also described in Japanese Patent Provisional Publications No. 58(1983)-113926, No. 58(1983)-113927, No. 58(1983)-113926, No. 59(1984)-90841, No. 58(1983)-111936, No. 62(1987)-99751, No. 60(1985)-143331, No. 60(1985)-143332, No. 61(1986)-14630, No. 62(1987)-6251, No. 63(1988)-220238, No. 63(1988)-151618, No. 63(1988)-281149, No. 59(1984)-133542, No. 59(1984)-45438, No. 62(1987)-269958, No. 63(1988)-305343, No. 59(1984)-142539, No. 62(1987)-253159, No. 62(1987)-266538, No. 63(1988)-107813, No. 64(1989)-26839, No. 62(1987)-157024, No. 62(1987)-192036, No. 1(1989)-297649, No. 2(1990)-127635, No. 1(1989)-158429, No. 2(1990)-42, No. 2(1990)-24643, No. 1(1989)-146033, No. 2(1990)-838, No. 2(1990)-28638, No. 3(1991)-109539, No. 3(1991)-175440, No. 3(1991)-121443, No. 2(1990)-73245, No. 3(1991)-119347, U.S. Patents No. 4,636,461, No. 4,942,120, No. 4,269,927, No. 4,900,652, No. 4,975,354, European Patent Publication No. 0355568A2, and Japanese Patent Applications No. 2(1990)-326222, No. 2(1990)-415037, No. 2(1990)-266615, No. 2(1990)-43791, No. 3(1991)-160395, No. 2(1990)-142635, No. 3(1991)-146503 and No. 4(1992)-77261. The silver halide emulsion of the present invention is advantageously used in photographic materials disclosed in Japanese Patent Provisional Publications No. 62(1987)-269958, No. 62(1987)-266538, No. 63(1988)-220238, No. 63(1988)-305343, No. 59(1984)-142539, No. 62(1987)-253159, No. 1(1989)-131541, No. 1(1989)-297649, No. 2(1990)-42, No. 1(1989)-158429, No. 3(1991)-226730, No. 4(1992)-151649, Japanese Patent Application No. 4(1992)-179961 and European Patent Publication No. 0508398A1. EXAMPLE 1 In a reaction vessel, 1,200 ml of gelatin solution (containing 18 g of deionized alkaline treated bone gelatin having methionine content of about 40 µmol/g, pH: 4.3) was placed. To the vessel, 12 ml of Ag-1 solution (containing 20 g of silver nitrate, 0.8 g of the above-mentioned gelatin, 0.2 ml of 1N nitric acid in 100 ml) and 12 ml of X-1 solution (containing 16.9 g of sodium chloride, 0.8 g of the above-mentioned gelatin, 0.3 ml of 1N sodium hydroxide solution in 100 ml) were simultaneously added at the feed rate of 24 ml per minute while keeping the temperature at 43 °C. After the mixture was stirred for 2 minutes, 19 ml of Ag-2 solution (containing 2 g of silver nitrate, 0.8 g of the above-mentioned gelatin, 0.2 ml of 1N nitric acid in 100 ml) and 19 ml of X-2 solution (containing 1.4 g of potassium bromide, 0.8 g of the above-mentioned gelatin, 0.2 ml of 1N sodium hydroxide solution in 100 ml) were simultaneously added to the mixture at the feed rate of 31 ml per minute. After the mixture was stirred for 2 minutes, 36 ml of Ag-1 solution and 36 ml of X-1 solution were simultaneously added to the mixture at the feed rate of 48 ml per minute. Thus, grain nuclei were formed. The structure of the nuclei is AgCl|AgBr|AgCl. The difference in chloride content or bromide content between AgX 1 and AgX 4 or AgX 4 and AgX 3 was 100 %. Further, 20 ml of NaCl-1 solution (containing 10 g of sodium chloride in 100 ml) was added to the grain nuclei. The mixture was adjusted to pH 4.8, and heated to 75 °C. After the emulsion was ripened for 20 minutes, the emulsion was cooled to 60 °C, and adjusted to pH 5.0. Then, Ag-3 solution (containing 10 g of silver nitrate in 100 ml) and X-3 solution (containing 3.6 g of sodium chloride in 100 ml) were added to the emulsion according to a controlled double jet method while keeping the silver potential of 130 mV. The feeding rate at the start of the addition was 7 ml per minute. The rate was gradually increased by 0.1 ml per minute. Finally, 400 ml of Ag-3 solution was added to the emulsion. After a precipitating agent was added to the emulsion, the emulsion was cooled to 30 °C. The emulsion was precipitated, and washed with water. After a gelatin solution was added to the emulsion, the resulting emulsion was adjusted to pH 6.2 and pCl 3.0 at 38 °C. A part of the emulsion was sampled, and a replica of the grains of the emulsion was prepared. The replica was observed by a transparent electron microscope (TEM). The micrograph (about × 6,500) of silver halide grains is shown in Figure 1. The characteristics of the grains are described below. (1) The ratio of tabular grains having an aspect ratio (diameter per thickness) of not less than 2 to whole silver halide grains (% based on projected areas of the grains) is 91 %. (2) The chloride content of tabular grains having an aspect ratio of not less than 2 (mol %) is 99.25 %. (3) The tabular grains having an aspect ratio of not less than 2 have {100} major faces. (4) The thickness of tabular grains having an aspect ratio of not less than 2 (µm) is 0.29 µm. (5) The ratio of twinned crystal grains to tabular grains having an aspect ratio of not less than 2 (% based on projected areas of the grains) is 0 %. (6) The grain nuclei from which tabular grains having an aspect ratio of not less than 2 are formed have two gap interfaces of halide composition. (7) The distribution coefficient of relatively large tabular silver halide grains, said relatively large grains comprising 70 % of tabular silver halide grains having an aspect ratio of not less than 2 based on projected areas of the grains is 0.059. (8) The ratio of specific square tabular grains to tabular silver halide grains having an aspect ratio of not less than 2, said square tabular grains having such a square projected shape that a ratio of a long side to a short side is not more than 1.4 (% based on projected areas of the grains) is 94 %. Further, the average aspect ratio of tabular grains having an aspect ratio of not less than 2 is 3.4. The average diameter of tabular grains having an aspect ratio of not less than 2 is 1.0 µm. EXAMPLE 2 In a reaction vessel, 1,200 ml of gelatin solution (containing 20 g of deionized alkaline treated gelatin, pH: 4.3) was placed. To the vessel, 12 ml of Ag-1 solution and 12 ml of X-1 solution were simultaneously added at the feed rate of 50 ml per minute while keeping the temperature at 41 °C. After the mixture was stirred for 2 minutes, 22 ml of Ag-2 solution and 22 ml of X-2 solution were simultaneously added to the mixture at the feed rate of 62 ml per minute. After the mixture was stirred for 2 minutes, 38 ml of Ag-1 solution and 38 ml of X-1 solution were simultaneously added to the mixture at the feed rate of 50 ml per minute. Thus, grain nuclei were formed. The structure of the nuclei is AgCl|AgBr|AgCl. The difference in chloride content or bromide content between AgX 1 and AgX 4 or AgX 4 and AgX 3 was 100 %. Further, 23 ml of NaCl-1 solution was added to the grain nuclei. The mixture was adjusted to pH 5.0, and heated to 75 °C. After the emulsion was ripened for 20 minutes, the emulsion was cooled to 65 °C. A silver chloride fine grain emulsion (average grain size: 0.08 µm) was added to the emulsion at the AgCl feed rate of 4×10 -3 mol per minute for 10 minutes. Further, the fine grain emulsion was added to the emulsion for 70 minutes at the accelerating feed rate of 10 -4 mol per minute. After the addition, the emulsion was ripened for 8 minutes. After a precipitating agent was added to the emulsion, the emulsion was cooled to 30 °C. The emulsion was precipitated, and washed with water. After a gelatin solution was added to the emulsion, the resulting emulsion was adjusted to pH 6.2 and pCl 3.0 at 38 °C. A part of the emulsion was sampled, and a replica of the grains of the emulsion was prepared. The replica was observed by a transparent electron microscope (TEM). The micrograph (about × 5,000) of silver halide grains is shown in Figure 2. The characteristics of the grains are described below. (1) The ratio of tabular grains having an aspect ratio (diameter per thickness) of not less than 2 to whole silver halide grains (% based on projected areas of the grains) is 92 %. (2) The chloride content of tabular grains having an aspect ratio of not less than 2 (mol %) is 99.59 %. (3) The tabular grains having an aspect ratio of not less than 2 have (100) major faces. (4) The thickness of tabular grains having an aspect ratio of not less than 2 (µm) is 0.22 µm. (5) The ratio of twinned crystal grains to tabular grains having an aspect ratio of not less than 2 (% based on projected areas of the grains) is 0 %. (6) The grain nuclei from which tabular grains having an aspect ratio of not less than 2 are formed have two gap interfaces of halide composition. (7) The distribution coefficient of relatively large tabular silver halide grains, said relatively large grains comprising 70 % of tabular silver halide grains having an aspect ratio of not less than 2 based on projected areas of the grains is 0.077. (8) The ratio of specific square tabular grains to tabular silver halide grains having an aspect ratio of not less than 2, said square tabular grains having such a square projected shape that a ratio of a long side to a short side is not more than 1.4 (% based on projected areas of the grains) is 82 %. Further, the average aspect ratio of tabular grains having an aspect ratio of not less than 2 is 4.5. The average diameter of tabular grains having an aspect ratio of not less than 2 is 1.0 µm. EXAMPLE 3 After the grain formation in Example 2, the emulsion was heated to 72 °C. The emulsion was further ripened for 10 minutes. A precipitating agent was added to the emulsion, and the emulsion was cooled to 30 °C. The emulsion was precipitated, and washed with water. After a gelatin solution was added to the emulsion, the resulting emulsion was adjusted to pH 6.2 and pCl 3.0 at 38 °C. A part of the emulsion was sampled, and a replica of the grains of the emulsion was prepared. The replica was observed by a transparent electron microscope (TEM). As a result, about 38 % (based on the projected area) of the tabular grains having an aspect ratio of not less than 2 have the specific structures shown in Figures 3a and 3b. COMPARISON EXAMPLE 1 Silver halide grains are formed in the same manner as in Example 11B of European Patent Publication No. 0534395A1. A precipitating agent was added to the emulsion, and the emulsion was cooled to 30 °C. The emulsion was precipitated, and washed with water. After a gelatin solution was added to the emulsion, the resulting emulsion was adjusted to pH 6.2 and pCl 3.0 at 38 °C. A part of the emulsion was sampled, and a replica of the grains of the emulsion was prepared. The replica was observed by a transparent electron microscope (TEM). As a result, the shapes of the grains are similar to those shown in Figure 5 of European Patent Publication No. 0534395A1. The characteristics of the grains are described below. (1) The ratio of tabular grains having an aspect ratio (diameter per thickness) of not less than 2 to whole silver halide grains (% based on projected areas of the grains) is 65 %. (2) The chloride content of tabular grains having an aspect ratio of not less than 2 (mol %) is 100 % (3) The tabular grains having an aspect ratio of not less than 2 have {100} major faces. (4) The thickness of tabular grains having an aspect ratio of not less than 2 (µm) is 0.13 µm. (5) The ratio of twinned crystal grains to tabular grains having an aspect ratio of not less than 2 (% based on projected areas of the grains) is 19 %. (6) The grain nuclei from which tabular grains having an aspect ratio of not less than 2 are formed have no gap interfaces of halide composition. (7) The distribution coefficient of relatively large tabular silver halide grains, said relatively large grains comprising 70 % of tabular silver halide grains having an aspect ratio of not less than 2 based on projected areas of the grains is 0.17. (8) The ratio of specific square tabular grains to tabular silver halide grains having an aspect ratio of not less than 2, said square tabular grains having such a square projected shape that a ratio of a long side to a short side is not more than 1.4 (% based on projected areas of the grains) is 53 %. Further, the average aspect ratio of tabular grains having an aspect ratio of not less than 2 is 10.0. The average diameter of tabular grains having an aspect ratio of not less than 2 is 1.3 µm. Evaluation of the emulsions The emulsions of Examples 1 to 3 and Comparison Example 1 were sensitized in the following manner. After the emulsions were heated to 40 °C, the following spectral sensitizing dye was adsorbed on the silver halide grains. The amount of the dye was 65 % of the saturated amount. After the emulsions were further heated to 55 °C, a sodium chloride solution was added to the emulsions to adjust pCl of 2.0. Then, hypo (2×10 -5 mol per 1 mol of silver) and chloroauric acid (10 -5 mol per 1 mol of silver) were added to the emulsions. After the emulsions were ripened, the following antifogging agent (10 -3 mol per 1 mol of silver) was added to the emulsions. Then, the emulsions were cooled to 40 °C. A viscous agent and a coating aid were added to the emulsions. Each of the emulsions and a protective layer was coated on a triacetyl cellulose film base. The film was dried to obtain samples. The coated samples were exposed to light through minus blue filter (transmitting light of 520 nm or more) for 10 -2 minute. The samples were developed, and immersed in a development stopping solution and then in a fixing solution. The samples were then washed with water and dried. The photographic quality was evaluated as sensitivity per granularity. The quality is improved by increasing the value of the sensitivity or decreasing the value of the granularity. As a result, the evaluated value of Example 1 is 115, the value of Example 2 is 118, the value of Example 3 was 121, and the value of the Comparison Example 1 was 100. Thus, the superior quality of the present invention was confirmed. EXAMPLE 4 In a reaction vessel, 1,200 ml of gelatin solution (containing 25 g of deionized alkaline treated bone gelatin having methionine content of about 30 µmol/g, pH: 4.3) was placed. To the vessel, Ag-41 solution (containing 20 g of silver nitrate in 100 ml) and X-41 solution (containing 16.9 g of sodium chloride and 0.8 g of the above-mentioned gelatin in 100 ml) were simultaneously added at the feed rate of 50 ml per minute for 20 seconds while keeping the temperature at 40 °C. After the mixture was stirred for 30 seconds, 53 ml of X-42 solution (containing 0.73 g of potassium bromide and 0.36 g of sodium chloride in 100 ml) was added to the mixture at the feed rate of 60 ml per minute. After the mixture was stirred for 2 minutes, Ag-41 solution and X-41 solution were simultaneously added to the mixture at the feed rate of 50 ml per minute for 40 seconds. Thus, grain nuclei were formed. The structure of the nuclei is AgCl|AgBr 0.5 Cl 0.5 |AgCl. The difference in chloride content or bromide content between AgX 1 and AgX 4 was 50 %. Further, 13 ml of NaCl-1 solution was added to the grain nuclei. The mixture was adjusted to pH 5.0, and heated to 75 °C. After the emulsion was ripened for 20 minutes, Ag-3 solution and X-3 solution were added to the emulsion according to a controlled double jet method while keeping the silver potential of 140 mV. The feeding rate at the start of the addition was 7 ml per minute. The rate was gradually increased by 0.1 ml per minute. Finally, 400 ml of Ag-3 solution was added to the emulsion. A part of the emulsion was sampled, and a replica of the grains of the emulsion was prepared. The replica was observed by a transparent electron microscope (TEM). The characteristics of the grains are described below. (1) The ratio of tabular grains having an aspect ratio (diameter per thickness) of not less than 2 to whole silver halide grains (% based on projected areas of the grains) is 93 %. (2) The chloride content of tabular grains having an aspect ratio of not less than 2 (mol %) is 99.39 %. (3) The tabular grains having an aspect ratio of not less than 2 have (100) major faces. (4) The thickness of tabular grains having an aspect ratio of not less than 2 (µm) is 0.2 µm. (5) The ratio of twinned crystal grains to tabular grains having an aspect ratio of not less than 2 (% based on projected areas of the grains) is 0 %. (6) The grain nuclei from which tabular grains having an aspect ratio of not less than 2 are formed have two gap interfaces of halide composition. (7) The distribution coefficient of relatively large tabular silver halide grains, said relatively large grains comprising 70 % of tabular silver halide grains having an aspect ratio of not less than 2 based on projected areas of the grains is 0.06. (8) The ratio of specific square tabular grains to tabular silver halide grains having an aspect ratio of not less than 2, said square tabular grains having such a square projected shape that a ratio of a long side to a short side is not more than 1.4 (% based on projected areas of the grains) is 95 %. Further, the average aspect ratio of tabular grains having an aspect ratio of not less than 2 is 5.5. The average diameter of tabular grains having an aspect ratio of not less than 2 is 1.2 µm. After the addition was completed, the emulsion was ripened for 5 minutes. A precipitating agent was added to the emulsion. A coating sample was prepared from the emulsion in the same manner as in Examples 1 to 3 and Comparison Example 1. The samples was then evaluated in the same manner as in the sensitometry of Examples 1 to 3 and Comparison Example 1. As a result, the evaluated value (sensitivity per granularity) of Example 4 was 125.

Claims: A silver halide emulsion comprising silver halide grains, which are dispersed in a dispersing medium, wherein at least 60 % of the silver halide grains based on projected areas of the grains are tabular grains having an aspect ratio of not less than 2, said tabular silver halide grains having a chloride content in the range of 20 to 99.99 mol %, {100} major faces, and a thickness of not more than 0.35 µm, and wherein twinned crystal grains contained in the emulsion comprise not more than 10 % of the tabular silver halide grains based on projected areas of the grains, and the tabular silver halide grains are formed from grain nuclei, which have at least one gap interface of halide composition. The silver halide emulsion as claimed in claim 1, wherein the tabular silver halide grains have such a grains size distribution that relatively large grains have a distribution coefficient of not more than 0.11, said relatively large grains comprising 70 % of the tabular silver halide grains based on projected areas of the grains. The silver halide emulsion as claimed in claim 1, wherein at least 47 % of the tabular silver halide grains based on projected areas of the grains have such a square projected shape that a ratio of a long side to a short side is not more than 1.4. The silver halide emulsion as claimed in claim 1, wherein at least 10 % of the tabular silver halide grains based on projected areas of the grains have such a rectangular projected shape that one angle is rounded compared with the other three angles. The silver halide emulsion as claimed in claim 1, wherein at least 70 % of the silver halide grains based on projected areas of the grains are the tabular grains. The silver halide emulsion as claimed in claim 1, wherein the tabular silver halide grains have a chloride content in the range of 50 to 99.9 mol %. The silver halide emulsion as claimed in claim 1, wherein the tabular silver halide grains have a thickness in the range of 0.05 to 0.30 µm. The silver halide emulsion as claimed in claim 1, wherein the tabular silver halide grains have an aspect ratio in the range of 2 to 25. The silver halide emulsion as claimed in claim 1, wherein twinned crystal grains contained in the emulsion comprise not more than 8 % of the tabular silver halide grains based on projected areas of the grains.

Inventors: SAITOU MITSUO, SAITOU, MITSUO, SAITOU, MITSUO, C/O FUJI PHOTO FILM CO., LTD.
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1994
Main classification: G03C1
IPC codes: G03C1